1. Field of the Invention
The present invention relates to an impact-sensor mounting structure for an automotive vehicle and, more particularly, to a structure for mounting an impact sensor at a front portion of a vehicle body for sensing a crash of the automotive vehicle with a desired degree of responsiveness.
2. Description of the Related Art
Recent years, there is the growing tendency that an air bag system is loaded in automotive vehicles as one of security measures. The air bag system is so arranged as to absorb a crash energy acting upon the passenger or passengers by expanding the air bag upon crash of the automotive vehicle. Hence, it is essential to mount a sensor capable of sensing a crash of the automotive vehicle with high accuracy while securing a desired degree of responsiveness.
For conventional air bag systems, it is general that the sensor system comprises a plurality of front sensors mounted in a so-called crash zone at the front portion of the vehicle body and one cowl sensor mounted to a cowl panel, etc. within the vehicle compartment. This sensor system is generally arranged in such a fashion that the cowl sensor is set to be highly sensitive while the front sensors are set to be less sensitive in order to prevent them from sensing too sensitive. Further, the air bag system is provided with an AND circuit so arranged as to accept two signals indicative of a car crash from at least one of the front sensors and the cowl sensor and generate a signal for expanding the air bag.
Accordingly, it is extremely significant in expanding the air bag upon crash of a car how accurate the impact sensor system can sense the car crash, i.e. how accurate the impact sensor system can respond to the impact of the crash of the vehicle body.
Description will now be made of a representative example of an impact sensor 10 of a mass damping type with reference to FIG. 17. As shown in FIG. 17, reference numeral 10a denotes a sensing mass (a ball) made of a conductive material capable of sensing upon crash, reference numeral 10b stands for a cylinder defining a passage for the sensing mass 10a, reference numeral 10c denotes a terminal so arranged as to apply electricity as the sensing mass 10a rolls to come into abutment with the sensing mass 10a, and reference numeral 10d denotes a magnet so arranged as to hold the sensing mass 10a at the end portion of the cylinder 10b under normal circumstances. It is to be noted herein that the responsiveness of the sensor system depends upon a roll of the sensing mass 10a. In order to improve the responsiveness of the sensor system, it may be understood that the axial direction of the sensing mass 10a is so arranged as to be perpendicular to a crash object, i.e. the direction in which the sensor system senses the crash is so set as to be horizontal with respect to the vehicle body.
However, the front sensors of the conventional sensor system mounted at the front portion of the vehicle body cannot achieve high responsiveness particularly when the vehicle body is broken in the front portion thereof upon a crash and the directional property of the vehicle body is lost as the result of a deformation of the vehicle body.
In order to solve this disadvantage, Japanese Patent Laid-open Publication (kokai) No. 50(1985)-141,045 discloses an impact sensor arranged so as to gain a constant degree of responsiveness within a particular range of angles by adapting the direction of sensing a crash by the impact sensor to the direction in which acceleration is caused to occur at a particular range of the angles due to the crash and by operating an equal degree of acceleration at any angle. Further, Japanese Utility Model Laid-open Publication (kokai) No. 2(1990)-10,159 discloses an impact sensor system whose forward mounting section is set to be more rigid than its rearward mounting section with the attempt to lose no directional property of the sensor itself even due to the deformation of the front portion of a front frame of the vehicle body upon a car crash, thereby avoiding a decrease in responsiveness. Furthermore, Japanese Patent Laid-open Publication (kokai) No. 63(1988)-291,752 discloses an impact sensor which is mounted to the vehicle body in such a state that it descends forwards, thereby taking advantage of a vertical component of acceleration, which is caused to occur upon a severe crash, as well as a horizontal component of acceleration, in order to react against a crash with high sensitivity.
The impact sensor disclosed in Japanese Patent Laid-open Publication No. 50-141,045 is so complex in structure that its production costs become too expensive and the impact sensor itself is pivotally disposed to sense an impact from a certain range of directions so that a wide space is required to cover the certain range of the directions.
On the other hand, the impact sensor disclosed in Japanese Utility Model Laid-open Publication (kokai) No. 2-10,159 is said to be effective when a front frame of the vehicle body is compressed and crashed in the axial direction thereof, however, it does not present the great advantages if the front frame thereof would be compressed and deformed in an upward or downward direction as the result of a crash.
Further, the impact sensor disclosed in the Japanese Patent Laid-open Publication No. 63-291,752 is mounted to the vehicle body in a forwards descending manner in order to take advantage of the vertical component of acceleration. This prior patent publication is silent about an influence of deformation of the frame due to a car crash upon responsiveness of the impact sensor, so that it neither discloses nor implies whatsoever any idea of improving responsiveness of the impact sensor by paying attention to the deformation of the frame or the direction of input of external force upon crash of the vehicle body.